Apparatus for rotating a transducer assembly of a borehole logging tool in a deviated borehole

ABSTRACT

A borehole logging tool having a dipole acoustic energy source and at least one spaced-apart dipole acoustic energy receiver is lowered into a deviated borehole. A motor rotates the source and receiver about the axis of the borehole tool. A first signal is produced representing source and receiver rotational position relative to a vertical direction. A second signal is produced representing a fixed reference level. A comparator produces a third signal of the difference between the first and second signals. The motor rotates so as to minimize such third signal, thereby maintaining the focus of the source and receiver in a horizontal direction that is perpendicular to the direction of eccentricity of the borehole tool with respect to the borehole axis.

BACKGROUND OF THE INVENTION

It has long been known to acoustically log open wellbores to determinethe velocities of compression ("P") waves and shear ("S") wavestraveling through rock formations located in the wellbore region.Logging devices have been used for this purpose which normally comprisea sound source (i.e., transmitter) and one or more receivers disposed atpreselected distances from the sound sources. The use of remotelyspaced, multiple receivers is intended to aid in distinguishing betweenvarious arriving wave fronts since travel time differentials increasewith increasing distance from the transmitter, as described in U.S. Pat.No. 4,383,308 to R. C. Caldwell.

Such arriving wave fronts generally include both headwaves and guidedwaves. A first arriving event is the headwave commonly called acompressional wave which represents acoustic energy which has beenrefracted through the formation adjacent the wellbore. Thiscompressional wave travels as a fluid pressure wave in the wellbore mudfrom the transmitter to the formation where it travels at thecompressional wave velocity of the particular formation. Thecompressional wave then travels to the receiver through the wellbore mudas a fluid pressure wave.

A second arriving event is the headwave commonly called a shear wavewhich is also refracted through the formation adjacent the wellbore.Unlike the compressional wave, the shear wave travels at shear velocitythrough the formations. The particles of the formation along the path ofpropagation are vibrated in a direction perpendicular to the directionof the propagation of the wave.

A third arriving event is the guided wave commonly called a tube wave orStoneley wave which causes a radial bulging and contraction of theborehole and its travel is, therefore, associated with the boreholewall, that is, the boundary between the borehole fluids and theformation solids.

A fourth arriving event is the guided wave commonly called a normalmode, pseudo-Rayleigh wave, or reflected conical wave. The travel ofthis normal mode is restricted to the borehole and has an oscillatorypattern normal to its direction of travel. Normally, the shear wave isindistinguishable from the onset of this normal mode due to concurrentarrival times.

Various signal timing and wave front analysis methods have also beensuggested for distinguishing between these various wave fronts receivedat a given receiver. Most of these methods involve timing circuits whichanticipate the receipt of, and facilitate the collection of, such wavefront information. For descriptions of various logging techniques forcollecting and analyzing acoustic wave data, please refer to U.S. Pat.No. 3,333,238 (Caldwell); U.S. Pat. No. 3,362,011 (Zemanek,Jr.); ReissueU.S. Pat. No. 24,446 (Summers); and U.S. Pat. No. 4,383,308 (Caldwell).

In the design of such acoustic logging tools, various types oftransmitters, such as piezoelectric or magnetostrictive transmitters,have been suggested for creating the acoustic logging signals. Forconventional logging operations, most such transmitters have beencentrally located in the borehole, and have been adapted to generatesound which is radiated in a multidirectional (360°) pattern from thetransmitter to adjacent wellbore surfaces. Such transmitters are wellsuited for creating compression waves in surrounding rock and sandformations.

Recently, attention has been directed to developing transmitters whichare particularly suited to shear wave logging. Such transmittersgenerally attempt to achieve a single point force application of soundenergy to the borehole wall. The theory behind point force transmittersis that they are capable of directly generating shear waves.Conventional multidirectional transmitters are said to be capable onlyof indirectly creating shear waves. Point force type transmittersproduce shear waves of substantially higher amplitudes than heretoforepossible with conventional multidirectional compression wavetransmitters. Accordingly, formations such as loosely consolidated orunconsolidated sand, for which shear waves cannot be refracted back intothe hole to permit definitive detection using conventional compressionwave receivers, may now be shear wave logged with these shear wavelogging systems. U.S. Pat. No. 4,649,525 to Angona and Zemanek, Jr.describes a shear wave acoustic logging system employing such a pointforce transmitter for the shear wave generation.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a boreholelogging tool for acoustic shear wave logging of a subsurface formationpenetrated by a deviated borehole. A dipole acoustic energy source ismounted for rotation about the axis of a borehole logging tool so as tofocus the directivity of acoustic energy such that it is perpendicularto the axis of the borehole tool. At least one axially spaced-apartdipole acoustic energy receiver is mounted for rotation about the axisof the borehole tool to focus the reception of acoustic energy in thesame direction as the focusing of the source. A motor is mechanicallycoupled to the source and receiver for rotating the source and receiverabout the axis of the borehole tool. A first signal is producedrepresenting the source and receiver rotational positions relative to avertical direction. A second signal is produced representing a fixedreference level. A comparator produces a third signal of the differencebetween such first and second signals. This third signal is applied tothe motor to cause it to rotate the source and receiver to a positionthat minimizes the third signal so as to maintain the focus of thesource and receiver in a horizontal direction that is perpendicular tothe direction of eccentricity of the borehole tool with respect to theborehole axis.

In one aspect, the first signal is produced by a first potentiometerhaving a stator and a rotor, the stator being coupled to a gimbalmounted weight so that it moves in response to the vertical pull ofgravity on the weight, and the rotor being rotationally coupled with thesource and receiver. The second signal is produced by a secondpotentiometer coupled to a fixed voltage source. The third signal isproduced by an operational amplifier that subtracts such first andsecond signals.

In another aspect, the first signal is produced by a digital shaftencoder fixed to a gimbaled mounted weight so that it moves in responseto the vertical pull of gravity on the weight and rotatably coupled tothe source and receiver. The second signal is produced as a fixeddigital reference number. The third signal is produced by amicroprocessor that subtracts such first and second signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a borehole logging tool eccentrically positionedwithin a cased unconsolidated formation for conducting a shear waveacoustic logging of such formation in accordance with the presentinvention.

FIG. 2 is a top view of the borehole logging tool and casedunconsolidated formation of FIG. 1 showing the directions for theasymmetrical acoustic energy action of the dipole source and for thesensitivities of the dipole receiver of FIG. 1.

FIG. 3 illustrates receiver signal amplitudes with the dipole sourceaction and the receiver sensitivities of FIG. 1 acting in the samedirection at various angles to the direction of tool eccentricity withinthe borehole as illustrated in FIG. 2.

FIGS. 4-6 are electrical schematics of portions of the borehole loggingtool of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In unconsolidated subsurface formations, borehole logging tools haveused a dipole acoustic energy source and a plurality of spaced-apartdipole acoustic energy receivers to generate and record the flexuralmode associated with a pure bending of the borehole whose low frequencypart propagates at the shear velocity of the unconsolidated formation.In such an unconsolidated formation, shear velocity is less than theacoustic velocity of the borehole fluid and the logging tool provides adirect measure of the shear velocity in contrast to the borehole loggingtools employing monopole acoustic energy source and receivers.

However, a great portion of the boreholes are cased with steel liningand cement. In such a cased borehole the flexural mode, i.e., shearwave, generated by a dipole acoustic energy source in an unconsolidatedformation is often obscured by a high amplitude tube wave traveling at avelocity close to the acoustic velocity of the borehole fluid.

An off-center, or eccentrically positioned dipole acoustic energy sourcegenerates several modes, among which is the tube wave. In an open hole,i.e., uncased, the tube wave is strongly coupled to an unconsolidatedformation and propagates at a velocity close to that of the shear wavevelocity of the unconsolidated formation. This results in interferenceswhich do not critically interfere with the determination of the shearwave velocity (see "Eccentric dipole sources in fluid-filled boreholes:Numerical and experimental results", by Leslie and Randall, J. Acoust.Soc. Am., Jun. 1990, pgs. 2405-2421). Mainly the amplitude is altered asit may not decrease monotonically with increasing source-to-receiverdistance. However, in cased boreholes, the tube wave propagates at avelocity close to the acoustic velocity of the borehole fluid.Therefore, for a cased unconsolidated formation, the tube wave willalways arrive before the shear wave, thereby interfering with itsdetermination.

It is therefore an object of the present invention to provide for theacoustic shear wave logging of a subsurface unconsolidated formationsurrounding a cased borehole that minimizes such tube wave interferenceon the detection of shear waves.

Referring now to FIG. 1, there is illustrated a borehole logging tooluseful in carrying out the present invention. An unconsolidatedformation 10 is penetrated by a borehole 14 which is deviated from thevertical and is lined with steel well-casing 12 and cement 11. Aborehole logging tool 13 is suspended within the deviated borehole 14 bycable 15. The force of gravity causes the borehole tool 13 to restagainst, or in juxtaposition with, the well casing 12 in the case of adeviated or horizontal borehole. A dipole acoustic energy source 16generates asymmetrical acoustic energy which travels by way of the fluidwithin the borehole and the surrounding cased formation ascompressional, tube and shear waves to a plurality of spaced-apartdipole acoustic energy receivers 17.

To carry out the present invention with the borehole logging tool ofFIG. 1, the off-centered borehole tool 13 is oriented, as shown in FIG.2, so that the directions of focus for the asymmetrical acoustic energyaction of the dipole source 16 and for the sensitivities of the dipolereceivers 17 are perpendicular to the axis of the borehole tool as wellas perpendicular to the direction of tool eccentricity. Suchconfiguration allows the present invention to minimize the effect ofinterfering tube waves on the recorded shear waves since both the sourceand receivers point in the same direction (i.e. parallel directions) andsuch direction is perpendicular to the direction of eccentricity of thelogging tool within the borehole. Such minimizing effect can be seen inFIG. 3 wherein the amplitude of the recorded tube wave is minimized withboth source action and receiver sensitivity directed perpendicular tothe direction of tool eccentricity which has been set equal to zero. Itcan be further seen that the tube wave amplitude decrease phenomenon isefficient within a deviation of about ±20° around the directionperpendicular to that of the eccentricity (e.g. 90°). It is noted thatthe directions of source action and receiver sensitivity with respect tothe direction of eccentricity (i.e. 0 °) are illustrated along theordinate of FIG. 3. In contrast, tube wave amplitude is maximized whenboth the source action and receiver sensitivities are directed parallelto the direction of tool eccentricity (e.g. 0° or 180°).

FIG. 3 is illustrative of recorded acoustic waves within a borehole of6.35 cm. inner radius and a 1.3 cm. offset of the source and receiver.The source and receiver spacing is 12 meters and the source has a centerfrequency of 3 kHz. The steel casing is 0.80 cm. thick and the cementsheath is 3.645 cm. thick.

In a further aspect, the amplitude of the tube wave increases comparedto that of the compressional and shear waves, whatever the orientationof the source and receivers, within increasing values of logging tooleccentricity within the borehole. The same phenomenon is observed withincreasing borehole radius. Also, with a shorter source-to-receiverspacing or a more ringing source wave form of longer duration in time,the effects of the tube wave will be enhanced so that the shear wavesignal can be totally obscured. It is important to note that a highamplitude tube wave leads to an increase of the dynamic range of thewhole signal so that the shear wave signal may not be detectable in thepresence of noise.

Similar results have been obtained when changing the casing, the cement,and the formation characteristics. When the formation is fast (i.e., itsshear wave velocity is greater than the acoustic velocity of theborehole fluid) the tube wave arrives later than the shear wave and doesnot obscure it.

Referring again to FIG. 1, there is shown a rotating motor assembly 21for rotating the source 16 and the receivers 17 so that they will actalong a direction perpendicular to the direction of eccentricity.Signals from the borehole tool 13 are transmitted uphole by conductorswithin cable 15 to any suitable utilization system at the surface of theearth. For example, the utilization system is illustrated as comprisingan uphole analysis and control circuit 23 and a recorder 24 in orderthat the output from circuit 23 may be correlated with depth asindicated from a depth measuring sheath 25.

The dipole acoustic energy source 16 and the dipole acoustic energyreceivers 17 may preferably take the form of the bender-type describedin the aforementioned U.S. Pat. No. No. 4,049,525 to Angoona andZemanek, Jr., the teaching of which is incorporated herein by reference.The bender disc is highly directional with peak sensitivity in thedirection perpendicular to the disc face. In the present invention, suchdiscs will be mounted with their faces in the same direction parallel tothe axis of the tool and will be rotated by the downhole gyroscope andmotor so as to be perpendicular to the direction of tool eccentricitywithin the borehole as detected by the downhole sensor.

Having described the apparatus of the present invention in conjunctionwith FIG. 1, the rotating motor assembly 21 that rotates the source 16and receivers 17 so that they are focused perpendicular to the axis ofthe logging tool and to the direction of eccentricity of the loggingtool in the borehole will now be described in detail in conjunction withFIGS. 4-6.

Referring to FIG. 4, a motor 30 is rotatably coupled, as shown by dashedlines 27 and arrows 22, to acoustic energy source 16 and to acousticenergy receivers 17. Sensor 31 rotates along with source 16 andreceivers 17, as shown by dashed line 25 and arrow 20, and outputs asource and receiver rotational position-to-vertical signal RPTV.Comparator 33 determines the difference between signal RPTV and a fixedreference signal REF and rotates motor 30 so as to minimize suchdifference and thereby maintain the focus of the source and receivers ina horizontal direction which, in the case of a deviated borehole withthe logging tool lying in juxtaposition with the lower side of theborehole from the force of gravity, is in a direction perpendicular tothe axis of the borehole tool and to the direction of eccentricity ofthe logging tool within the borehole.

Rotating motor assembly 21 may operate in accordance with the presentinvention in either an analog mode as shown in FIG. 5 or in a digitalmode as shown in FIG. 6.

Referring firstly to FIG. 5, there is illustrated an analogservomechanism mode of operation. Sensor 31 comprises a gimbaled mountedweight 40 which mechanically positions, by way of coupling 41 the statorof a potentiometer 42 to provide a vertical reference. The rotor ofpotentiometer 42 is positioned as shown by dashed line 25 and arrow 20,by the rotational positioning of the source 16 and receivers 17. Thisserves to provide a signal RPTV which is a receiver rotationalposition-to-vertical reference. A voltage proportional to thisrotational position of the rotor of potentiometer 42 is input tocomparator 33 through the resistor networks 47 and 48 to the invertinginput of difference amplifier 45. A reference voltage REF is provided bypotentiometer 46 and is input to comparator 33 through the resistornetwork 43 and 44 to the non-inverting input of difference amplifier 45.Amplifier 45 determines the difference of the two potentiometer outputsto produce an analog signal on line 49 which represents the differenceof the receiver rotational position-to-vertical signal RPTV and thevoltage reference signal REF. This analog difference signal is appliedto amplifier 50 which drives motor 30 through current amplifyingtransistors 51 and 52 in a direction to minimize the difference betweenthe signals RPTV and REF. Any difference detected by amplifier 45 isamplified to improve the response speed of the operation.

Referring now to FIG. 6, there is illustrated a digital servomechanismmode of operation. Sensor 32 comprises a digital shaft encoder 61 whoserotor is mechanically coupled to, and rotates with, the motor 30, thesource 16, and receiver 17 (as shown by dashed line 25 and arrow 20).The stator of digital shaft encoder 61 is mechanically moved by thegimbal mounted weight 60 to insure proper positioning regardless oflogging tool inclination. The resulting output of encoder 61 is areceiver rotational position-to-vertical reference signal RPTV which isan absolute 8-bit binary word indicating the degree of inclination ofthe source and receivers relative to the vertical. This signal RPTV isinput to the P1 port of an 8-bit microcontroller 62 as the transducerrotational error and is subtracted from a reference number set to fullscale (i.e., 11111111B). The resulting output at port P2 is input to an8-bit digital-to-analog convertor 64 which converts the microcontroller62 output to an analog voltage that is amplified by operationalamplifier 65 and associated resistors 66-69 and transistors 70-71. Thisamplified analog signal is applied to motor 30 for rotationalpositioning the source 16 and receivers 17 so as to return anyrotational error to zero. Initially the system is set up such that theencoder 61 is at half full scale with the source and receivers set atthe desired rotational position for horizontal motion.

Microcontroller 62 determines the difference between the rotationalerror signal RPTV and the reference number in accordance with thefollowing assembly code designed for an Intel 8751/87C51 microcontrollerwith on-board eprom:

    ______________________________________                                        ORG 00H                                                                       START          :     start point at 00h (8751 resets                                               to this point),                                          SJMP MAIN      :     jump to main start-of-code and                                                start,                                                   ORG 30H        :     begin code beyond interrupts,                            MAIN           :     heart of code starts here,                               MOV A, #11111111B                                                                            :     put full-scale reference level in                                             accumulator,                                             SUBB A,P1      :     subtract transducer position                                                  encoder from reference (pro-                                                  duces bipolar output from                                                     DAC input),                                              MOV P2,A       :     move result to dac/motor                                                      output port,                                             SJMP MAIN      :     go back to start of code and                                                  repeat,                                                  END                                                                           ______________________________________                                    

It is to be understood that the circuit components illustrated in FIGS.5 and 6 are merely representative of alternate embodiments of thepresent invention. Particularly with respect to the embodiments of FIGS.5 and 6, various types and values of circuit components may be utilized.In accordance with such embodiments the following sets forth specifictypes and values of the circuit components.

    ______________________________________                                        Reference Designation                                                                            Description                                                ______________________________________                                        DC Gear Motor 30   Globe #C43A113-1                                           Potentiometers 42 & 46                                                                           Ohmite, 1K                                                 Operational Amp. 45 & 65                                                                         Texas Inst., TL084                                         Transistors 51 & 70                                                                              Motorola, 2N5192                                           Transistors 52 & 71                                                                              Motorola, 2N5195                                           Digital-to-Analog Conv. 64                                                                       Datel, UP8PC                                               Microcontroller 62 Intel, 8751                                                Optical Shaft Enc. 61                                                                            B.E.I., #5V70                                              Resistors 43 & 47  Dale, RN55/60C (10K)                                       Resistors 44, 48, 66 & 67                                                                        Dale, RN55/60C (100K)                                      Resistors 68 & 69  Dale, RN55/60C (200K)                                      Oscillator 59      12 MHZ                                                     ______________________________________                                    

Having now described a preferred embodiment of the present invention, itwill be apparent to those skilled in the art of acoustic well loggingthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as set forth in the appendedclaims.

We claim:
 1. A borehole logging tool for acoustic shear wave logging ofan unconsolidated subsurface formation penetrated by a deviatedborehole, comprising:a) a dipole acoustic energy source mounted forrotation about the axis of the borehole tool to focus the directivity ofacoustic energy such that it is perpendicular to the axis of theborehole tool, b) at least one axially spaced-apart dipole acousticenergy receiver mounted for rotation about the axis of the borehole toolto focus the reception of acoustic energy in the same direction as thefocusing of said source, c) a motor mechanically coupled to said sourceand said receiver for rotating said source and said receiver about theaxis of the borehole tool, d) means for providing a first signalrepresenting the source and receiver rotational positions relative to avertical direction, e) means for providing a second signal having afixed reference level representing said vertical direction. f) acomparator for providing a third signal of the difference between saidfirst and second signals representing the difference between said firstand second signals, and g) means for applying said third signal to saidmotor to cause it to rotate said source and said receiver to a positionthat minimizes said third signal so as to maintain the focus of saidsource and said receiver in a horizontal direction that is perpendicularto the direction of eccentricity of the borehole tool with respect tothe borehole axis.
 2. The borehole logging tool of claim 1 wherein:a)said means for providing said first signal comprises a firstpotentiometer having a stator and a rotor, said stator being coupled toa gimbal mounted weight so that it moves in response to the verticalpull of gravity on said weight, and said rotor being rotationallycoupled with said source and receiver, whereby said potentiometerprovides a voltage output representing the rotational position of saidsource and said receiver relative to a vertical direction, and b) saidmeans for providing said second signal comprises a second potentiometercoupled to a fixed voltage source.
 3. The borehole logging tool of claim2 wherein said comparator is an operational amplifier that subtractssaid first signal represented by voltage output of said firstpotentiometer from said second signal represented by a fixed voltageoutput of said second potentiometer to provide said third signal.
 4. Theborehole logging tool of claim 3 wherein said means for applying saidthird signal to said motor comprises a current amplifying means forrotating said motor in a direction to minimize the difference betweensaid first and second signals.
 5. The borehole logging tool of claim 1wherein said means for providing said first signal comprises a digitalshaft encoder fixed to a gimbal mounted weight so that it moves inresponse to the vertical pull of gravity on said weight and rotatablycoupled to said source and said receiver, the rotational position ofsaid digital shaft encoder providing an output representing therotational position of said source and said receiver relative to avertical direction, said means for providing said second signal providesa fixed digital reference number.
 6. The borehole logging tool of claim5 wherein said comparator is a microcontroller programmed to subtractsaid first signal represented by the output of said digital shaftencoder from said second signal represented by said fixed digitalreference number to provide said third signal.
 7. The borehole loggingtool of claim 6 wherein said means for applying said third signal tosaid motor comprises:a) a digital-to-analog convertor, and b) a currentamplifying means for rotating said motor in a direction to minimize thedifference between said first and second signals.